Gas hydrate is a special type of inclusion compound which forms when light hydrocarbon (C.sub.1 --C.sub.4) constituents and other light gases (CO.sub.2, H.sub.2 S, N.sub.2 etc) physically react with water at elevated pressures and low temperatures. Natural gas hydrates are solid materials and they do not flow readily in concentrated slurries or solid forms. They have been considered as an industrial nuisance for almost sixty years due to its troublesome properties of flow channel blockage in the oil/gas production and transmission systems. In order to reduce the cost of gas production and transmission, the nuisance aspects of gas hydrates has motivated years of hydrate inhibition research supported by oil/gas industry. (Handbook of Natural Gas, D. Katz etc., pp. 189-221, McGraw-Hill, N.Y., 1959; Clathrate Hydrates of Natural Gases, E. D. Sloan, Jr. Marcel Dekker, Inc. 1991). The naturally occurring natural gas hydrates are also an interest as an alternative energy resource for the industry. (International Conferences on Natural Gas Hydrates, Editors, E. D. Sloan, Jr., J. Happel, M. A. Hnatow, 1994, pp. 225-231-Overview: Gas Hydrates Geology and Geography, R. D. Malone; pp. 232-246-Natural Gas Hydrate Occurrence and Issues, K. A. Kvenvolden).
Since natural gas hydrates contain as much as 180 standard cubic feet of gas per cubic foot of solid natural gas hydrates, several researchers have suggested that hydrates can be used to store and transport natural gases. (B. Miller and E. R. Strong, Am. Gas. Asso. Mon 28(2), 63-1946). The high concentration of gas in the hydrates have led researchers to consider intentionally forming these materials for the purpose of storing and transporting natural gases more cost/effectively and safely. U.S. Pat. No. 5,536,893 to Gudmundsson discloses a multi-stage process for producing natural gas hydrates. See also Gudmundsson et al., "Transport of Natural Gas as Frozen Hydrate", ISOPE Conf. Proc., V1, The Hague, NL, June, 1995; "Storing Natural Gas as Frozen Hydrate", SPE Production & Facilities, Feb. 1994.
U.S. Pat. No. 3,514,274 to Cahn et al. teaches a process in which the solid hydrate phase is generated in one or a series of process steps, then conveyed to either storage, or directly to a marine transport vessel requiring conveyance of a concentrated hydrate slurry to storage and marine transport. Pneumatic conveyance of compressed hydrate blocks and cylinders through ducts and pipelines has also been proposed. See Smirnov, L. F., "New Technologies Using Gas Hydrates", Teor. Osn. Khim. Tekhnol., v 23(6), pp. 808-22 (1989), application WO 93/01153, Jan. 21, 1993.
Based upon the published literature (E. D. Sloan, 1991 Clathrate Hydrates of Natural Gases, Marcel Dekker), transporting of a concentrated gas hydrate slurry in a pipe from stirred-tank vessel would appear to be incompatible with reliable operation, or even semi-continuous operation. The blockage of pipes, and fouling of the reactors and mixing units are the critical issues. The searching of chemical/mechanical method to prevent gas hydrate blockage/fouling is still the focus of the current gas hydrate research. (Long, J. "Gas Hydrate Formation Mechanism and Kinetic Inhibition", PhD dissertation, 1994, Colorado School of Mines, Golden, Colorado; E. D. Sloan, "The State-of-the-Art of Hydrates as Related to the Natural Gas Industry", Topical Report GRI 91/0302, June, 1992; Englezos, P., "Clathrate Hydrates", Ind. Eng. Chem. Res., V32, pp. 1251-1274, 1993).
Gas hydrates are special inclusion compounds having a crystalline structure known as a clathrate. Gas molecules are physically entrapped or engaged in expanded lattice of water network comprising hydrogen-bonded water molecules. The structure is stable due to weak van der Waals' between gas and water molecules and hydrogen-bonding between water molecules within the cage structures. Unit crystal of structure I clathrate hydrates comprise two tetrakaidecahedron cavities and six dodecahedron cavities for every 46 water molecules, and the entrapped gases may consist of methane, ethane, carbon dioxide, and hydrogen sulfide. The unit crystal of structure II clathrate hydrates contain 8 large hexakaidecahedron cavities and 16 dodecahedron cavities for every 136 water molecules.
Clathrate hydrates occur naturally in permafrost or deep-ocean environments, thus are considered an important natural resource. Utilizing such a resource requires understanding of gas hydrate formation and dissociation. "Kinetics of Methane Hydrate Decomposition," Kim et al., Chemical Engineering Science, Vol. 42, No. 7, pp.1645-1653 (1987) discusses the kinetics of methane hydrate decomposition, indicating that pressure dependence further depends on the difference in gas fugacities at equilibrium pressure and decomposition pressure. "A Multi-Phase, Multi-Dimensional, Variable Composition Simulation of Gas Production from a Conventional Gas Reservoir in Contact with Hydrates," Burshears et al., Unconventional Gas Technology Symprouis of the Society of Petroleum Engineers, pp. 449-453 (1986), discusses dissociation of hydrates by depressurization without an external heat source. "Hydrate Dissociation in Sediment" Selim et al., 62d Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, pp. 243-258 (1987) relates rate of hydrate dissociation with thermal properties and porosity of the porous media. "Methane Hydrate Gas Production: An Assessment of Conventional Production Technology as Applied to Hydrate Gas Recovery," McGuire, Los Alamos National Laboratory, pp.1-17 (1981) discusses feasibility of hydrate gas production by both thermal stimulation and pressure reduction. "Gas Hydrates Decomposition and Its Modeling", Guo et al., 1992 International Gas Research Conference, pp. 243-252 (1992), attributes difference in chemical potential as the driving force for hydrate dissociation.
U.S. Pat. No. 2,375,559 to Hutchinson et al., entitled "Treatment of Hydrocarbon Gases", discloses a method of forming hydrates by cooling and dispersing the components when combining the components. Similarly, U.S. Pat. No. 2,356,407 to Hutchinson, entitled "System for Forming and Storing Hydrocarbon Hydration", discloses hydrate formation using water and a carrier liquid. U.S. Pat. No. 2,270,016 to Benesh discloses hydrate formation and storage using water and alcohol, thereby forming blocks of hydrate to be stored.
U.S. Pat. No. 3,514,274 to Cahn et al. discloses transportation of natural gas as a hydrate aboard ship. The system uses propane or butane as a carrier. U.S. Pat. No. 3,975,167 to Nierman discloses undersea formation and transportation of natural gas hydrates. U.S. Pat. No. 4,920,752 to Ehrsam relates to both hydrate formation and storage wherein one chamber of a reservoir is charged with hydrate while another chamber is evacuated by decomposition of hydrate into gas and ice.
Hydrates, much like ice, are good insulators. The process taught in the Cahn et al. '274 patent, stores hydrates in a liquid hydrocarbon slurry, thus enabling the liquid hydrocarbon handles to act as a heat transfer agent. But storing and transporting hydrates in their solid form is inherently more efficient because without the liquid component of the slurry, more natural gas (in its hydrate form) can be stored in a given volume.
In recovering gas from gas hydrate, it is also economically advantageous to maintain the above volumetric efficiency, thus favoring minimization of the volume of heat transfer agent needed to supply the hydrate's large heat of dissociation (410 kJ/kg for methane hydrate, approximately 25% higher than ice's heat of melting. Ref: Clathrate Hydrates of Natural Gases, E. D. Sloan, Jr. Marcel Dekker, Inc. 1991).